The object of this proposed research is to investigate the mechanism of adhesion of platelets to collagen fibers. The proposal is divided into two, interrelated projects. The first project involves an investigation of the structural features of the collagen fiber responsible for the adhesion of platelets. Our first additional approach will be to reinvestigate the role of ionizable, polar residues in collagen, keeping in mind that ionizable residues are involved in the formation of fibers and that native fibers are required for the adhesion of platelets. Chemical modification of polar residues of polymeric collagen will be carried out, the collagen will be bound to Sepharose, and assays of platelet adhesion and release will be determined by our recently-developed assay system. The second project involves an attempt to isolate and identify the protein(s) of the platelet membrane which results in the binding of platelets to collagen. Membrane proteins will be labeled by various means and solubilized in non-ionic detergents. The isolated proteins will be incubated with fibrous collagen, and the binding protein will be isolated and characterized.